Airtran $158 RT LGA-ATL-LGA
Delta $269 RT HPN-ATL-HPN
Continntal $176.50 RT EWR-ATL-EWR
Assuming your 16-20 age profile is correct, you have to ask yourself if going on CO out of EWR just to capture the FF miles is worth the extra $18.50, especially if you're alot of FF miles away from a reward ticket.
Assuming it's not, you then have to ask yourself if it's worth $111 of convenience to fly DL out of HPN versus Airtran out of LGA. If it is, you have your answer. If it's not worth the $111, especially if you're paying for this trip with your own money, I'd head to LGA....
($111 will buy some fun in ATL...)
ALL views, opinions expressed are mine ONLY and are NOT representative of those shared by Southwest Airlines Co.